The antiviral and antigrowth activity of interferon (IFN) makes this agent a promising approach to cancer therapy. Three tumors that are unusually sensitive to IFN-a are chronic myelogenous leukemia (CML) in the benign phase, hairy-cell leukemia, and the T-cell lymphomas associated with mycosis fungoides. Studies using partially purified IFN-a or IFN-A2a (Roferon-A) therapy have shown that most patients with benign-phase CML achieve hematologic remission. Furthermore, in some patients, long-term therapy results in hematologic remission and complete suppression of the Philadelphia chromosome. Early intervention, within six months after diagnosis, increases the response rate among CML patients treated with IFN-A2a. IFN-a, IFN-A2a (Roferon-A) and IFN-A2b (Intron A) are remarkably active against hairy-cell leukemia, with similar results achieved in clinical studies. The question of antibody formation in trials of recombinant IFN-a products will be discussed in this paper. Neutralizing antibody development and antibody positivity as determined by enzyme-linked immunosorbent assay (ELISA) have been reported with both IFN-A2a and IFN-A2b (Intron A), but the rate reported with IFN-A2b was significantly lower. However, possible differences in the sensitivity of the assays have been mentioned. The effects of differing dosing regimens also were noted. Additional trials conducted by clinical investigators using identical methods will help answer questions arising from these discrepancies and determine the full value of interferon in cancer therapy.